The present invention relates to a system for controlling ignition timing of an internal combustion engine such as an automotive engine, and more particularly, to a system for preventing knocking of the engine at a high altitude.
The ignition timing is advanced with an increase of engine speed. When the load on the engine, which is represented by pressure of intake air, is low, charging efficiency of the intake air in cylinders reduces so that combustion speed decreases. Therefore, it is further necessary to advance the ignition timing in accordance with increase of the intake air pressure.
The ignition timing is determined in accordance with the intake air pressure (absolute pressure) at wide-open throttle as shown by a solid line in FIG. 5. When the engine is operated at a low altitude where the atmospheric pressure is 760 mmHg, the ignition timing at the wide-open throttle is set at a point a. For example, as the atmospheric pressure decreases to 660 mmHg at a higher altitude, the ignition timing at the wide-open throttle is advanced to a point b. However, the intake air charging efficiency at the high altitude is not reduced in proportion to the reduction of the atmospheric pressure. Accordingly, it is necessary to further correct the ignition timing for the high altitude.
Japanese Patent Application Laid Open Nos. 60-43178 and 60-81466 disclose ignition timing control system at the high altitude. Since the engine power decreases at the high altitude, the ignition timing is advanced taking into consideration of intake air charging efficiency reduction.
However, at the high altitude, the pressure of exhaust gas decreases with the atmospheric pressure so that the amount of the exhaust gas from the cylinders during the valve overlap period reduces. Accordingly, when the engine is operated at the same intake air pressure (absolute pressure) the intake air charging efficiency increases compared with that at low altitude. Therefore, if the ignition timing is advanced by the angle which is controlled by the same intake air pressure as the low altitude, the timing is excessively advanced, which causes knocking of the engine.